1. Field of the Invention
This invention relates to a method and apparatus for reducing carbon monoxide (CO) and hydrocarbon (HC) emissions in the exhaust of an internal combustion engine, and more particularly, to a new and improved method and apparatus for introducing oxygen enriched air into the intake of an internal combustion engine during at the least the initial start-up and warming-up periods of the engine after the engine is started.
2. Background of the Invention
A significant portion of the total pollutants emitted by light-duty passenger vehicles occurs immediately following the start-up of the engine. During this period, the engine block and exhaust manifold are cold, and the catalytic converter has not yet reached high conversion efficiencies (generally the catalyst is not fully effective at temperatures below about 250.degree. C. for CO and 250.degree. C. to 340.degree. C. for HC emissions conversion). Moreover, it is a common practice to operate spark ignition engines with richer fuel-air mixtures during initial start-up and warming-up periods for proper operating driveability and acceleration. As a result, more unburned and partially burned hydrocarbons and more carbon monoxide are present in the exhaust after the engine is started and within the first 20 seconds of idling. On the other hand, the nitrogen oxides (NO.sub.X) emissions in the exhaust tend to be low during this start-up period, but increase significantly when the vehicle is first accelerated. Consequently, the characteristics of the emissions in the exhaust of a spark ignition engine are influenced by both the engine operating conditions and the heating characteristics of the catalytic converter.
In order to meet the California Air Resources Board (CARB) Transitional Low Emission Vehicle (TLEV), Low Emission Vehicle (LEV) and Ultra Low Emission Vehicle (ULEV) standards, substantial reductions are required for HC and CO emissions from a spark ignition engine during the cold phase of the federal test procedure cycle. Similarly, many light-duty passenger cars are required to reduce these emissions to comply with Tier-II (year 2004) standards. This growing concern over start-up/cold-phase emissions has led to various attempts to develop new emissions treatment techniques that reduce the HC and CO levels in the exhaust emissions.
The attempts to reduce cold-phase (i.e., the first 505 seconds of federal test procedure driving cycle, as specified in the Code of Federal Regulations, Title 40, Part 86, Subpart 8, revised 1993) emissions by after-treatment methods can be grouped into three broad categories: (1) thermal management of the catalytic converter, including low-mass manifolds, double-walled exhaust pipes, electrically heated catalysts, exhaust-gas burners, exhaust-gas igniters, and insulated converters (with vacuum or refractory material); (2) placement of the converter closer to the exhaust manifold; and (3) management of the interaction between the hydrocarbons and the catalyst, using hydrocarbon adsorbent or traps in the exhaust. However, durability, fuel penalty, additional capital costs, unwanted heat in the engine compartment, and the complexity of these systems limit their application in vehicles.
On the other hand, a potentially attractive alternative is to control the emissions at the source itself (i.e., during combustion). One type of in-cylinder emission control is to introduce oxygen enriched air instead of ambient air to the air intake of the engine. Use of such oxygen enriched air can potentially reduce CO and HC emissions from a spark ignition engine, even during start-up and warming up periods because oxygen enrichment of the intake air reduces the emissions from the engine rapidly (even when the engine is cold). As a result, it helps to minimize the converter limitations during the cold phase and should improve converter efficiency. This method has the advantage of fewer add-on components, of lesser mechanical complexities, of not altering the fuel economy of the engine, and of an easier to modify system (the air intake system is easier to modify than the exhaust system).
Even though the oxygen-enrichment of the intake air in spark ignition engine powered vehicles results in the lowering of cold-phase HC and CO emissions in the exhaust of the vehicle, it tends to result in an increase in NO.sub.X in the exhaust. To some extent, the increase in NO.sub.X in the emissions has been offset by newer NO.sub.X control technologies that can remove nitrogen oxides from the emissions. These technologies include lean NO.sub.X catalysts and the injection into the exhaust gases of monatomic nitrogen induced by a pulse arc (see, for example, U.S. patent application Ser. No. 08/019,102, filed on Feb. 18, 1993 and assigned to the same assignee of record as the present application). The lack of an economical source of on-line oxygen equipment has made it difficult to provide a practical application of this concept. Recent developments of relatively compact oxygen-enrichment devices, such as selectively permeable membranes, has made oxygen-enrichment potentially practical. Nevertheless, it is necessary to have a simple, compact mechanical system driven by the engine itself that will economically extract oxygen from the air before an oxygen-enriched air intake system can practically be used on a vehicle.
Accordingly, it is an object of the present invention to provide a new and improved method and apparatus for reducing HC and CO emissions in the exhaust of a spark ignition internal combustion engine by introducing oxygen-enriched air into the air intake of a spark ignition engine vehicle during a short period of time following the start-up of the engine.
It is another object of the present invention to provide a new and improved method and apparatus for introducing oxygen-enriched air into the air intake of a spark ignition engine vehicle by diverting the intake air through a selectively permeable membrane for a predetermined time period so that oxygen enriched air is supplied to the engine intake manifold at least during a short period of time following the start-up of the engine.
It is yet another object of the present invention to provide a new and improved method and apparatus for introducing oxygen-enriched air into the air intake of a spark ignition engine vehicle by diverting the intake air through a selectively permeable membrane for a predetermined time period so that oxygen enriched air is supplied to an air plenum and from the air plenum into the engine intake manifold at least during a short period of time following the start-up of the engine.